Presses for terminating electrical terminals to wires typically employ linear pneumatic or rotary electric actuators to provide the forces necessary for crimping a terminal to a wire. Because these actuators are often energy inefficient, difficult to control from a quality standpoint, and are particularly susceptible to maintenance problems, a distinct group of presses employing the use of electromagnets has been developed. In a general design scheme of such magnetic presses, a pair of electromagnets is connected to an electrical circuit, which circuit is operative to supply electrical current to the magnets' windings in a way that induces attractive magnetic fields. One of the magnets is operatively connected to a displaceable shaft which transmits forces to a crimping tool, which, in turn, transmits crimping forces to a terminal thereby crimping the terminal to an electrical wire. Magnetic presses are advantageously capable of generating compressive forces in the order of several tons of crimping pressure, but problems have arisen regarding the control of such forces.
Prior devices which address the control of magnetic presses of the foregoing design are disclosed in U.S. Pat. No. 3,584,496('496) and U.S. Pat. No. 3,783,662('662). Referring first to the '496 patent, two circuits are therein described. The first circuit defines an embodiment applying a pulse of current from a power source to the windings of a pair of magnets. One of the magnets is stationary, and the other magnet is reciprocable and is attached to a tooling shaft. The pulse has been predefined in current and amplitude based on prior experience with a particular work piece. The circuit does not provide for a sensor or feedback system to control the current sent to the magnets. The second circuit results in application of a constant crimp force through the use of a feedback system including a force transducer, e.g. a piezoelectric device or strain gauge. The force transducer is strategically placed to sense the force applied to an anvil of the crimp tooling. The force transducer is operative to send a proportional electrical signal to a comparator which compares the transducer signal to a reference signal, if there is a differential between the signals, the comparator then sends a control signal to the power source to modify the power input to the magnets until the transducer signal sufficiently approximates the reference signal. A timing circuit then controls the interval of time, i.e. the dwell time, that the crimping force is applied to the terminal, which time is equal to a predetermined interval of time. At the end of the dwell time, the terminal has been crimped, the magnets are de-energized, and the reciprocable magnet is returned, under a spring force, to an original position in preparation for the next crimp cycle.
The device described in the '662 patent is an improvement over the '496 device in that a let down circuit has been added for the purpose of limiting the initial current to the magnets, thereby controlling the velocity of the crimp tooling and avoiding excessive kinetic energy in the tooling on the down stroke. After the tooling makes the initial contact with a work piece, the current supplied to the magnets is increased for generating sufficient crimping forces. A transducer/comparator circuit, such as described above in respect of the '496 patent, is used to control the force applied during the dwell time. When the reference signal is met by the transducer signal value, the power to the magnets is cut off, and the reciprocating magnet returns to an undisplaced position in preparation for the next cycle.
A disadvantage of the foregoing magnetic presses is that magnetic flux fields exist between the magnets even after the power signal to the magnets has been zeroed. This occurs because the electromagnetic material does not return to its original state, i.e. an insubstantial magnetic flux, but, rather, after removal of the circuit induced magnetic field a residual magnetism inheres in the electromagnetic material. Such residual magnetism results in a continuation of the forces of attraction between the magnets, thereby retarding their relative separation in preparation for the next crimp cycle, and, thereby disadvantageously resulting increased cycle time. Moreover, the use of a transducer to sense the pressure of the crimp tooling and send a control signal to a comparator for processing adds delay in response time of the overall control system. Furthermore, the use of a transducer increases the capital equipment and maintenance expenses of the prior devices. Additionally, when the magnet is returned under the force of the spring the magnet will tend to come to an abrupt stop, i.e. slam, into an abutment on the up-stroke thereby potentially damaging the component parts of the press. A further disadvantage of the prior devices is that they are not adapted to receive standard application tooling with an automatic terminal feed mechanism.
In view of the above, what is needed is a magnetic press which has a minimum cycle time, avoids slamming on the up-stroke, is adapted to receive standard application tooling with an automatic terminal feed mechanism, and is inexpensive to manufacture.